1. Field of the Invention
The present invention relates to a display system for displaying information in the viewfinder of a camera, allowing the object to be seen in the viewfinder and capable of superposing photographing data in a pattern on the image of the object.
2. Description of the Prior Art
As shown in FIG. 11, a distance measuring area A, a light measuring area B and photographing data C are displayed within the viewfinder of a camera. The photographing data C is displayed in static images or flickering images to enable the photographer to recognize the photographing data C visually only when necessary. Accordingly, the transmittance of the background of the photographing data C may be low provided that the photographing data C form a high contrast to the background. However, when the photographing data is displayed within the focusing screen of the viewfinder, the photographing data is displayed on a background having a high transmittance, because the object is displayed in a dark image on the focusing screen and it is difficult to recognize the image clearly if the background has a low transmittance. Furthermore, the photographer is unable to recognize the photographing data precisely if the photographing data is not displayed in high contrast with the background.
The liquid crystal display (hereinafter abbreviated to "LCD") of a twisted nematic phase type used generally as a display has two polarizing plates. Since the polarizing plates intercept not less than 50% of the incident light, the LCD is unable to display bright images in the viewfinder. When the polarizing plates are exposed to a high temperature, the life of the same is shortened and the durability of the same is deteriorated. Accordingly, it is desirable to use a LCD not using any polarizing plates as a display for the viewfinder. An LCD of a guest-host type employs no polarizing plate. Generally, the LCD of a guest-host type employs a nematic liquid crystal (Np liquid crystal) having a positive anisotropic dielectric property (a property that the orientation factor goes perpendicular to the surfaces of the electrodes when a voltage is applied across the electrodes) as host (base material) and a dichroic coloring matter having an anisotropic light-absorption property as guest (admixture). This Np liquid crystal transmits light when the orientation factor is perpendicular to the surfaces of the electrodes (homeotropic orientation), and obstructs light when the orientation factor is parallel to the surfaces of the electrodes (homogeneous orientation). Therefore, the LCD employing an Np liquid crystal is a negative LCD which displays photographing data in a transparent pattern (a negative image) on an opaque background when a voltage is applied thereto. To display photographing data in a positive pattern (a positive image) on the negative LCD, a voltage is applied continuously to the back electrode, i.e., the electrode of the background sheet, and a voltage is applied to or removed from the front electrode, i.e., the electrode of the viewing sheet, to control the light transmittance of the LCD so that the photographing data is visible in an opaque image on the transparent background.
Although the light transmittance of the negative LCD is higher than that of the LCD employing polarizing plates, the transparency of the negative LCD is not sufficiently high and the opacity of the opaque portion (a portion in which the molecules are in a homogeneous orientation) of the same is insufficient and hence the contrast between the indication and the background is low because the orientation factor is not arranged in a perfect homeotropic orientation when a voltage is applied across the electrodes. Although an electrode for maintaining the background transparent is formed around leads connected to the transparent electrodes of the pattern, the leads are visible together with the pattern of photographing data because the voltage is removed from the leads when the pattern is displayed.
U.S. Pat. No. 4,572,617 proposes a method for enhancing the transparency of the LCD to increase the contrast between the pattern of photographing data and the background. This method mixes a surface active agent in the liquid crystal to enhance the homeotropic orientation to keep the background opaque, and tubs the front electrode in one direction with cloth for orientation to increase the opacity when the molecules are arranged in a homogeneous orientation. This method requires different orientation processes respectively for the front electrode and the back electrode, which increases the steps of manufacturing process and requires difficult processes.
Another method proposed to increase the contrast between the pattern of photographing data and the background holds two liquid crystal layers between transparent electrodes of the same shape with the orientations of the orientation factors perpendicular to each other. Such an arrangement of the liquid crystal layers requires three glass sheets, which increases the thickness of the LCD and requires accurate alignment of the corresponding patterns of the four transparent electrodes.